pack n play

Hi guys, I just ordered the pack n play Barlow and illneed to use it as a crib for a few months. My son is 8 months old. Where exactly in it will he sleep? It comes with a mattress but from the pictures online the mattress seems to be at the very top and it's for younger babies. My son would definetly fall out if it if the mattress is this high. Can the mattress go in the bottom of the playpen? Is the mattress even necessary or does he sleep directly in the playepen